[Warning: The following contains spoilers from the Sherlock finale, "His Last Vow." Don't add this to your mind palace until you've seen the episode. You've been warned.]On the season finale of Sherlock on Sunday, the intrepid consulting detective was thrown ...

[Warning: The following contains spoilers from the Sherlock finale, "His Last Vow." Don't add this to your mind palace until you've seen the episode. You've been warned.]On the season finale of Sherlockon Sunday, the intrepid consulting detective was thrown for a loop not just once, but three times. Here's a rundown: Sherlock: An introduction to villain Charles Augustus Magnussen1. Mary, Mary, quite contrary: Mary (Amanda Abbington), the newlywed bride of John Watson (Martin Freeman), seemed so sweet and innocent... until Sherlock (Benedict Cumberbatch) stumbled across her holding blackmailer Charles Augustus Magnussen (Lars Mikkelsen) at gunpoint. And then she shot Sherlock in the torso. Apparently, Mary was once a trained assassin. John sure can pick 'em! 2. Magnussen's secret files: Magnussen's success as a ruthless blackmailer relied on having an extensive amount of information at his fingertips on all of his victims. But when Sherlock went to Appledore, assuming that all the files were downstairs in a huge vault, he was stunned to learn that like himself, Magnussen uses a mind palace to store all of that dirt and can recall everything at will. 3. Moriarty's resurrection? In the finale's closing minutes, every screen in England was hijacked with a broadcast of Moriarty's face, accompanied by the words, "Did you miss me?" As far as we knew, he had blown his brains out on that rooftop at the end of last season. We turned to series producers Steven Moffat and Sue Vertue as well as Abbington for more answers. Here's what they had to say: We see the possible return of Moriarty (Andrew Scott) at the end of the episode, even though we thought we saw him die last season. Why did you want to bring him back? Steven Moffat: We always knew what we were going to do with Moriarty. This has been the game plan since the end of Series 1 when we realized we had a crack on our hands with Andrew. This is something that the whole series has been building to. I won't say what it is we're doing, but we have plans. And I'm pretty bloody certain those plans will please people. Sherlock's Amanda Abbington on playing Mary and scoop on Mr. Selfridge Season 2You've already started putting together Season 4 and 5? Moffat: Yes. We knew what we were doing with Moriarty. We had a whole set of specific story ideas mostly for Series 4 but reaching into Series 5, which is because Mark [Gattis] and I happened to be chatting one day and as occasionally happens, we just ducked out of the rain and into the accountant's office. We found ourselves getting very animated about a set of ideas we could have. I think they're the best set of ideas we've ever had for Sherlock. So I'm very excited about that. Will Season 4 pick up immediately from this finale because it ended on such a cliff-hanger?Moffat: We're going to fully account for everything we've done, yes. There will be nothing left dangling. Knowing that Moriarty was going to come back in some form, was that foreshadowed with his presence in Sherlock's mind palace?Moffat: No, that's a separate thing. I did debate as to whether or not we should show that we'd got Andrew Scott in. But I love the idea of Moriarty being locked up in the mind palace. But also, I think it reminds you of Moriarty but wrong-foots you into absolutely confirming that he's not part of the show anymore. And then suddenly he is. Can you discuss casting actors Timothy Carlton and Wanda Ventham (Cumberbatch's real-life parents) to play Sherlock's parents? Sue Vertue: There's very little about Sherlock's past. Steven and Mark's theories that you could only get that kind of person, or that person would have that freedom if they had a happy childhood and normal parents. And again, which parents would produce something like Benedict Cumberbatch? Both his parents are great actors, so again it was natural. There was so much about the Holmes family this season. How much more will we get with Mycroft (Gatiss) and Sherlock's relationship? And their parents now that we've met them?Moffat: I don't know. As we need to really. I mean, the moments when Sherlock collides with the real world are good. And in the real world you have parents and you have family and all that, so those things are useful for us but it really is story-dependent or emotion-dependent of what we're trying to do at that time. If it's relevant. It's something else we can do. Vertue: I loved the smoking bit. [Laughs]Watch a mini episode of SherlockGoing forward, will we be seeing Mary again? Amanda Abbington: Yeah, Steven and Mark are sitting down furiously writing story lines. There's no reason why she shouldn't come back. I think she's part of the Sherlock family for the time being.I get that feeling from people who've seen it and watched it. They like that she's another dynamic. Mary's still pregnant with a baby girl. but you never know with Mark and Steven. Like they said that Moriarty was dead and yet at the end he's back. How did it feel to dress up in that black catsuit and fire a gun? Was that unusual for you?Abbington: I've never been asked to do that before, so it was a real joy. I didn't know what was going to happen. I knew that she had a secret and that it was something to do with Magnussen. I knew that, but filming the first two episodes I had no idea that she would shoot Sherlock and that she's was an assassin and CIA. I had no idea about that, which is good because I didn't play that, which helps. So when I got to shoot Ben, it was fantastic. It was really fun. Did you have gun training?Abbington: Yes, we had a firearms expert come in. The gun I used was the gun that Daniel Craig used in Skyfall. It was purely coincidental. Lee, this wonderful firearms guy, taught me to fire it, said, "You know that's James Bond's gun." I was like, "Don't say that!" I was walking around going, "I've got a Walther PPK." It's brilliant. It was a little suspect when she knew what a skip code was...Abbington: A lot of people did. They were like, "That's ridiculous. Why would she know that?" Wait and see.Vertue: You do see [Sherlock] look to her. There are certain [clues] where when you look at Mary and we did see those things earlier, like we hid "liar" in there. When you're putting it on the graphics, once you're trying to hide something, it always looks hugely there. "No, no, put it further back." Add Sherlock and other shows to your Watchlist now and never miss an episodeThe "A.G.R.A." on the flash drive, which is supposed to stand for Mary's real initials, is a reference to the canon's Agra treasure. Have you guys planned out what Mary's real name is? Moffat: It's Amanda Abbington. [Laughs] No, that's too much in the future. I don't have an answer for that one.Molly (Louise Brealy) was a big part of this season. She even briefly found love! Will there be more Molly-Sherlock interaction? Moffat: I think she's become quite an important part of the show. One of the difficulties in adapting Sherlock Holmes... [is that] there are no women in Sherlock Holmes, the original stories. You really don't have the female perspective on them. And they're slightly flat when they turn up. And yet, every time we bring a woman into Sherlock, it works hugely well. A female perspective on that friendship and on that man is incredibly telling, incredibly illuminating. So we've expanded hugely the part of Mrs. Hudson (Una Stubbs) because she didn't speak in the original. And we brought Molly in. What is fascinating about women interacting with Sherlock Holmes is that they see through him so much faster than John ever does. They get him like that [snaps fingers]. Mary knows when he's fibbing. She says, "You're fibbing. Stop it. I know you're lying." And Irene Adler (Lara Pulver), she just finds the off switch in 30 seconds. It's just fascinating to see. And Molly, the story of Molly is from crush to understanding far too well. What inspired the mustache antics at the beginning of the season. Was that written in or was it because Martin Freeman had grown an actual mustache?Vertue: No, it was written it, much to his annoyance. Did he have to grow one then?Vertue: No he didn't because we shot things out of sequence, so sometimes he had to have it and sometimes he wouldn't. People were really upset about the mustache. They were really pleased when it went. [Laughs] What did you think of the Sherlock finale? What are you theories about Moriarty's return? Share them in the comments below.